keep it runnung

I have a 86 CRX HF, 1.3 carb'd (151k and runnig strong). When first started it will run between 20 to 40 minutes, then die. After that It will run for only run for five minutes before giving out. It will idle @ 900 RPM then after five minutes jump to 1200 RPM then sputter out. I have rebuilt the carb, replaced the plugs, wires, fuel filter, and pump. There are no visible signs of cracked or lose vacumm lines. Just seeing if anyone had a similar problem, and knows what is going on.

Shawn